The area image sensor has photoelectric conversion elements consisting of a plurality of photodiodes and so forth (referred to as ‘pixels’ hereinbelow) arranged in the form of a lattice, converts a subject optical image that is imaged on the pixel placement face (imaging face) by means of an imaging lens to an electrical signal (voltage signal) with a size corresponding to the amount of light received from each pixel, and outputs electrical signals from each pixel in a predetermined order. The electrical signals (signals corresponding to the density of the image) from each pixel are stored in memory so as to be arranged in the light-receiving position of each pixel on the basis of the outputted order after being converted to a digital signal, whereby an electrical signal corresponding to the subject optical image is obtained.
FIG. 23 is a constitution of one pixel of the conventional CMOS area image sensor shown in JP-A 2001-036816, for example. One pixel is constituted by a photodiode PD that performs conversion to an electrical charge amount according to the received light amount and accumulates the electrical charge; a reset transistor M1 consisting of a FET (Field Effect Transistor) for discharging electrical charge remaining in the photodiode PD before starting exposure; a switching transistor M3 consisting of an FET for controlling the timing for reading to a signal line L for the charge that has accumulated in the photodiode PD (exposure end timing), an amplification transistor M2 consisting of a FET for amplifying a voltage signal (voltage signal of the cathode of the photodiode PD) based on the electrical charge when the electrical charge that has accumulated in the photodiode PD after the end of exposure is outputted to the outside by the signal line L.
The cathode of the photodiode PD is grounded and the anode is connected to the source of the reset transistor M1 and the gate of the amplification transistor M2. Further, the drain of the reset transistor M1 and the drain of the amplification transistor M2 are connected to a VDD power supply. The source of the amplification transistor M2 is connected to the drain of the switching transistor M3 and the source of the switching transistor M3 is connected to the signal line L. Further, the gate of the reset transistor M1 is connected to a reset line R and the gate of the switching transistor M3 is connected to an address line A.
Signal lines L for outputting an electrical signal (known as a ‘light reception signal’ hereinbelow) from a plurality of pixels that are arranged in columns on the right side of each column, for example, with a plurality of pixels arranged in a lattice shape are arranged and address lines A for inputting a signal (read signal) for controlling the read timing for reading a reception signal to a plurality of pixels arranged in the row below each column, for example, and reset lines R for inputting a signal (reset signal) for controlling the discharge timing of the remaining electrical charge are arranged. A plurality of A/D converters 101 are provided in correspondence with each signal line L below the imaging face and the lower end of each signal line L is connected to the corresponding A/D converter 101. The source of the switching transistor M3 of the plurality of pixels arranged in each column is connected to the corresponding signal line L.
Further, one end of each address line A and each reset line R is connected to a control portion 100 that controls the output of the read signal and reset signal. The gates of the switching transistors M3 of the plurality of pixels arranged in each row are connected to the corresponding address lines A and the gates of the reset transistor M1 of the plurality of pixels arranged in each row are connected to the corresponding reset lines R.
The imaging operation by the CMOS area image sensor is performed as follows.
Suppose that the row numbers from the pixel column of the highest row of the imaging face to the pixel column of the lowest row are 1, 2, . . . n, and the address numbers of the address lines corresponding to each row are 1, 2, . . . n, a vertical synchronization signal is used as a synchronization signal for controlling an exposure operation from the first row to the nth row, that is, an exposure operation equivalent to one screen and a horizontal synchronization signal is used as a synchronization signal for controlling the exposure operation for each row. When a vertical synchronization signal is inputted, a read signal and reset signal are outputted in sync with the horizontal synchronization signal to each row in order from the first row by means of the control portion 100. The plurality of pixels arranged in each row are reset (discharge of remaining electrical charge) by means of a reset signal after the light reception signal has been read to the A/D converter 101 via the signal line L by means of the read signal and the exposure is started, wherein the exposure operation is performed until the next read signal and reset signal are inputted.
Therefore, with this CMOS area image sensor, the exposure operation of the plurality of pixels arranged in each row is started by providing a time difference corresponding to a cycle Th of the horizontal synchronization signal and, when time that corresponds to a cycle Tv of the vertical synchronization signal has elapsed, the exposure operation is ended, whereupon the reception signal is read from each pixel, A/D converted by the A/D converter 101, and then outputted to an external frame memory via a shift register. Further, the time difference between the start of exposure of the uppermost row and the start of exposure of the lowermost row is a time corresponding substantially to the cycle Tv of the vertical synchronization signal and, therefore, the light reception signals of all the pixels that constitute an image of one frame are obtained after the time 2Tv corresponding to two cycles' worth of the vertical synchronization signal has elapsed after the start of exposure.
Further, the conventional CMOS area image sensor is the main cause of degradation of the pickup image (original image) on account of the structure of this sensor and processing to compensate for the image degradation is required in a circuit downstream of the CMOS area image sensor.
For example, because the area image sensor has a flat imaging face with a horizontal rectangular shape, when the light of the average light amount is irradiated on the imaging face by means of an imaging lens as will be described subsequently, there is the problem that the light amount does not enter the whole of the imaging face uniformly and the periphery of the pickup image is then darker than the center thereof, that is, the brightness distribution of the original image differs from the subject optical image.
FIG. 24 is a schematic diagram showing an imaging optical system of a digital camera in which an area image sensor IS is provided. According to FIG. 24, when light reaching the area image sensor IS via the center of the lens Z is examined, while the incident light A enters the center So of the image read area S of the area image sensor IS via the center of the lens Z, light B that enters at an angle θ with respect to the incident light A enters the peripheral part Sr of the image read area S. The light path length from the center of the lens Z to the image read area S grows longer as light reaches the periphery of the image read area and, therefore, supposing that the light amount at the center So of the image read area S is 1, the light amount at the peripheral part Sr of the image read area S is theoretically determined by cos4 θ. Thus, in the area image sensor IS with a flat imaging face, the light amount at the peripheral part of the image read area S is small in comparison with the light amount at the center So of the image read area S. This tendency is more pronounced the more compact the imaging device is rendered by making the distance to the image sensor from the lens short.
Further, FIG. 25 shows the distribution of light amounts in the image read area S. As shown in the same figure, in the image read area S, the light amount is maximum at the center corresponding with the optical center of the lens and grows smaller toward the peripheral part. More specifically, the light amount gradually decreases in moving further away from the center point O and, in a remote area at substantially the same distance from the center point O, the light amount is substantially the same. The light amount distribution in an X cross-section of the image read area S is expressed by a secondary curved line in which the center point O has the maximum light amount, and, at a point P1 on the X axis a distance Lx from the center point O, a light amount at which the maximum light amount is x %, for example, is produced, as shown in FIG. 25B. Further, the light amount distribution of the Y-axis cross-section is also, as shown in FIG. 25C, expressed by a secondary curved line for which the center point O has the maximum light amount and, at a point P2 on the Y axis a distance Ly from the center point O, a light amount at which the maximum light amount is y %, for example, is produced. If the image is outputted by reflecting the light amount distribution of such an area image sensor as is, the image grow dark toward the periphery.
Therefore, in the case of a conventional area image sensor, a variety of techniques enabling a substantially uniform brightness to be obtained over the whole area of the output image by correcting such a light amount distribution have been proposed. For example, it has been proposed that a DSP (digital signal processor) for correcting a digital signal be incorporated in the area image sensor and that correction be performed as a result of the DSP multiplying the output value of each light-receiving element by the reciprocal number value of the ratio with respect to the maximum light amount of the light amount at the points where the light-receiving elements are positioned.
For example, the light amount at point P1 shown in FIG. 25 has a maximum light amount of x % and, therefore, the reciprocal number value at point P1 is (100/x). Therefore, when the output values of the pixels arranged on points P1 of the image read region S are multiplied by the reciprocal number value, corrected values that are substantially the same as the maximum light amount are obtained, as shown in FIG. 26. As a result, by performing correction by multiplying the output value of each pixel by the reciprocal number value corresponding with each pixel, a substantially uniform brightness is obtained over the whole of the output image.
However, this method of multiplying reciprocal number values by using the DSP necessitates the allocation of the reciprocal number values to all the pixels and, therefore, there is the drawback that a memory comprising a correction table in which a multiplicity of reciprocal number values are stored must be provided, and so forth. Moreover, in this case, the greater the number of pixels, the larger the number of reciprocal number values and a large memory capacity is therefore required, which leads to an increase in costs.
In order to limit the memory capacity, as shown in FIG. 27, creating a correction table that corresponds only to the pixels in one quadrant of the image read area S and using this correction table by expanding same to the other quadrants may also be considered. With this method, although the memory capacity can be reduced to substantially ¼, it is hard to say that there will be an adequate cost reduction.
Further, as a method that makes it possible to obtain a uniform light amount over the whole area of the image read area S without correcting the output from the pixels, combining a so-called ND (neutral density) filter with reduced light transmission toward the center of the image read area S with an area image sensor has been proposed. That is, if this ND filter is disposed in the vicinity of the front face of the area image sensor, the light amount at the center of the image read area S can be compulsorily reduced by the ND filter and, therefore, the whole of the image read area S can be afforded a uniform light amount.
However, in this case, by integrating the light amount of the internal region of the image read area S with the light amount of the peripheral region by cutting the incident light, there is the inconvenience of reducing the output of the whole of the area image sensor.
Further, as mentioned earlier, a conventional CMOS area image sensor generates image data equivalent to one frame by performing the exposure operation for a time corresponding to the cycle Tv of the vertical synchronization signal for each row in order by providing a time difference corresponding to a cycle Th of the horizontal synchronization signal from the uppermost row to the lowermost row and, therefore, when the subject optical image moves to the right within the imaging face in accordance with the movement of the photographic subject, for example, there is a shift between the position of the photographic subject at the exposure timing at the top within the imaging face and the position of the photographic subject at the exposure timing at the bottom, whereby the pickup image is an image in which the photographic subject drifts to the right side as one approaches the lower side of the screen. The drift state of the photographic subject grows larger with increased speed of movement of the photographic subject and, in cases where the photographic subject moves at high speed, this constitutes a moving image and image distortion is generated.
In order to resolve this problem, making the shift in the exposure start timing of each row by shortening the cycle Th of the horizontal synchronization signal, for example, as small as possible may also be considered. However, when the frequency of the horizontal synchronization signal is increased, another problem arises that the electrical power consumed by the area image sensor increases as a result of the increase in the electrical power consumed by the A/D converter 101 and so forth.
As detailed earlier, a conventional CMOS area image sensor has factors that cause image degradation in the original image in that the pickup image grows darker toward the periphery even when the pickup image is small on account of the structure of the CMOS area image sensor and image distortion is readily produced in the pickup image with respect to a moving body.